LED drive mode can adopt resistance current limiting, linear voltage regulator and switching converter 3 types. The resistive current limiting scheme is applicable to low efficiency applications, so it has high efficiency requirements. This method is not used in automobile lighting with wide input voltage range. The linear voltage stabilizer is suitable for low current or LED forward voltage drop slightly lower than the power supply voltage, but there are also problems of efficiency and small input voltage range. Switching converter has the characteristics of flexible circuit topology, high efficiency and wide input voltage. Therefore, after taking into account the factors of working efficiency, installation size, static current, working voltage, noise and output adjustment, the drive circuit is usually operated by switching converter. Switching converter topology is divided into Buck, Boost and Buck-Boost. At present, LED is used in automobile lighting, and its driving power must be lead-acid battery.
Because the input voltage range of the battery is very different from the normal range, the Buck Boost topology is generally used to meet the voltage requirements of LED arrays. The dc gain (the ratio of output voltage to input voltage) of this circuit topology is related to duty ratio D(the ratio of opening time to cycle in a switching cycle). When the battery voltage is lower than the required voltage of LED, D > 0.5 is adjusted to make the circuit under the boost state. When the battery voltage is higher than the required voltage of LED, adjust D < 0.5, so that the circuit is in a step-down state. LED is a current - controlled element with a luminance proportional to the current flowing through it. If the LED is not driven by constant current, the brightness of the LED will change even if the voltage is constant when the current passing through fluctuates. To keep the brightness stable and reliable, leds need constant current to drive them, and they need to be able to control ripple current at an acceptable level in any case. Therefore, the output of LED drive circuit must be constant current output rather than constant pressure output.
The buck-boost circuit converts the battery's electrical energy to power the LED array. The sampling circuit samples the current flowing through the LED and sends the signal to the control circuit. The control circuit analyzes the sampling information and adjusts the duty ratio of switching tube in buck-boost circuit to ensure the LED current is constant. When the circuit is abnormal, the LED is not damaged by controlling the protection circuit to cut off the power. In general, LED drive circuits must meet the following requirements:
(1) lifting pressure function. When the input voltage or the voltage drop of the LED itself fluctuates, the output voltage is adjusted to meet the requirement of constant output current and ensure the stability and reliability of LED luminescence.
(2) high power conversion efficiency. In order to reduce the drive loss, save cost, reduce the battery charge times, and extend the battery life.
(3) brightness adjustment function. When the surrounding environment is very dark, the signal lamp usually does not need the maximum current drive. At this time, the driving current can be controlled to change the brightness of the LED, thus reducing the power consumption of the LED. PWM signal control is a common method to adjust drive current.
(4) complete protection circuit. All kinds of protection measures should be set up to protect themselves and LED reliable work. For example, low pressure lock, overvoltage protection, overheat protection, output open circuit or short circuit protection.
(5) good heat dissipation function. According to the thermal characteristics of LED, temperature is one of the important factors affecting LED work. When driving at night, the LED is on for a long time, so it must have good heat dissipation function to ensure the LED life and work reliably.
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